To see if Alex in the shop or not, go to Loren's restaurant, talk to Loren, check if Mat is working or not (There's a text in green that would say so).
If he's working at the restaurant, then Alex must be covering for him at her shop. So don't work at Lapizza, just go back to the map and the to the electronics store, Alex should be there.
If he's not working today, then you must work at Lapizza to advance your time there. He works there once every 3 days.
In short, your working days at the restaurant must be divisible by 3 in order for you to see Alex in her shop